JefferyH
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Proper usage and temperature adjustment of non-digital hotplates/heating mantles?
I'm looking at heating mantles preferably with both heating and stirring, the only issue is that the ones that have a digital temperature
display/control do not have stirring, and the ones that do have stirring do not have any digital displays.
When heating certain mixtures target temperatures have to be met, and it can be detrimental to the mixture if over heated. Having a thermometer stuck
in the mix the entire time is just inconvenient, and if the temperature is overshot then there may be no reversing product degradation.
Is there something I am missing? On these analog/manual heating mantles (with knobs & no display), how can I accurately adjust and dial in the
temperature. Do I just have to spend time getting to know which temperatures correspond with how far the knob is turned?
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Zephyr
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It may be inconvenient to always have a thermometer in a reaction, it's not very difficult, and I think that for the amateur it is preferable to
spending extra money for a digital hotplate. In regards to temperature control, I find it useful in sensitive reactions to use and oil/water bath, so
that is the temperature is too high, I can add some cool oil and simply turn down the hotplate.
Also, there are many hotplate/stirrers that are digital, although they are quite pricey.
http://www.ebay.com/itm/Fisher-Scientific-Digital-Hotplate-a...
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Dr.Bob
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Using the temperature of the hotplate will NOT give you any real useful data on the temperature of the reaction that you are heating. A 400C
hotplate will not heat water in a beaker over 100C but an exothermic reaction can go to 200C on a 100C hotplate. Most reactions will be 10-50 C
lower than the hotplate due to heat losses, but that is only a generalization. In organic chemistry, most reactions are temperature controlled by
picking a solvent with a BP of the desired temp, as that allows the Bp to set the temp, as long as the condenser is well cooled, that method is called
refluxing.
If you don't have a boiling solvent to set the temp, then you will want to have a therm in the reaction. Then there are many ways to control the
actual temp, manual tweaking via a knob or variac (for a heating mantle), electronic temp regulators, and other tricks. Most people heating mantles
along with a stir plate, and then set the temp via some sort of power or voltage regulation. Again, I always start low and SLOWLY move the temp up,
almost all systems have a lag time, so the temperature will slowly rise, but people routinely overshoot the temp, as once the heating starts, it will
keep raising the temp for a while, once the power is lowered, as there is a time delay for the heat to move from heater to flask. Hotplates and
flasks have a thermal mass, which means that conductive heating is quite slow.
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JefferyH
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If I understand you correctly, that goes into relative contradiction to my experiences with my [magnetic stirring] hot plate in the past. My digital
hot plate, which is fairly inexpensive, has a temperature sensing feedback loop in the base, and can accurately hold the temperature of the mixture at
this. For instance, I may set it to 150 degrees and then simultaneously stir and measure the internal temperature of the liquid, and the temperature
on the thermometer and the temperature on the base have always been relatively within 5 degrees. If I were to add ice to a mixture that is at 150
degrees, the temperature reading on the base will almost immediately, and then begin to heat back up again. Based on my observations the
temperature sensor seems to measure right at the surface where contact is made by the glass
Consequently, I have only used [flat bottomed] beakers on my hot plates for those such reactions, so temperature conductivity is not terribly
horrible.
Another simple example for instance, is when I have boiled water in a beaker. As the hot plate heats up, the instant (within 5-10 seconds) it measures
100 degrees, the water will begin to boil. Drop ice in, water stops boiling, temperature immediately drops. Another example; I have used it to MP/BP
verify certain substances. Various white solids from new suppliers have been tested by setting the hot plate to their [decomposition] melting
temperature. One time for a phase transfer catalyst for instance, the moment the hotplate read its melting/decomposition point, the powder began to
melt, smoke, and turn black, roughly within 5 seconds of the read out.
Maybe I am misunderstanding you but I have never had the hot plate's reading temperature be that much[10-50 C] significantly lower than the reaction
as it seems to use a built in feedback loop and constantly adjust.
I actually managed to find some digital heating mantles with these exact features, that I'll probably be checking out soon.
[Edited on 19-5-2014 by JefferyH]
A better question is which will heat faster. In the case of a distillation, sure the mixture can only be heated to the reflux temperature but the
amount of heat input will determine the speed of the distillation. I am inclined to believe analogue and digital hotplates heat by different
mechanisms; one with variable input power and the other with a feedback loop that rapidly flickers on and off at full power. If this is the case
analogue might offer more control over distillation speed.
[Edited on 19-5-2014 by JefferyH]
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Bert
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Frankly, learning manual control of reaction temperatures with a thermometer and a water or oil bath is a basic skill you should learn.
Yes, it will take a little experience with your equipment to get the hang of this. And you definitely shouldn't start the learning process on
synthesis of energetic materials, or any process with expensive/irreplaceable reagents or a critical outcome.
After you learn these techniques, there are cheap PID controls and thermocouples available... With an appropriate relay, these will switch your
electrical heating for you, and they "learn" the response curve of your system, anticipating when to switch on/off. I have seen such systems made from
ebay for under $50.00.
Of course, you will need to learn some very basic electronics technology to implement this. Which will ALLWAYS be a good thing to know.
Rapopart’s Rules for critical commentary:
1. Attempt to re-express your target’s position so clearly, vividly and fairly that your target says: “Thanks, I wish I’d thought of putting it
that way.”
2. List any points of agreement (especially if they are not matters of general or widespread agreement).
3. Mention anything you have learned from your target.
4. Only then are you permitted to say so much as a word of rebuttal or criticism.
Anatol Rapoport was a Russian-born American mathematical psychologist (1911-2007).
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JefferyH
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Quote: Originally posted by Bert  | Frankly, learning manual control of reaction temperatures with a thermometer and a water or oil bath is a basic skill you should learn.
Yes, it will take a little experience with your equipment to get the hang of this. And you definitely shouldn't start the learning process on
synthesis of energetic materials, or any process with expensive/irreplaceable reagents or a critical outcome.
After you learn these techniques, there are cheap PID controls and thermocouples available... With an appropriate relay, these will switch your
electrical heating for you, and they "learn" the response curve of your system, anticipating when to switch on/off. I have seen such systems made from
ebay for under $50.00.
Of course, you will need to learn some very basic electronics technology to implement this. Which will ALLWAYS be a good thing to know.
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I was actually just looking into thermocouple systems, this could make up for the delays in heating with heat loss.
Do you by chance know how the digital/thermocouple systems work with regard to the voltage? Say I have a flask full of water with the thermocouple
submerged in the center. I want to boil it as fast as possible, ie, maximum energy input. I set the thermocouple-sensor to 110 degrees. The heating
mantle will continually heat the mixture trying to get it to 110 degrees, but because its water, the temperature will never be reached. Do you
know if when trying to heat to 110 degrees, if the flask will be utilizing maximum voltage/maximum energy input? Or will the voltage vary depending on
how far away from the target temperature it is?
Or would it be better to get an analogue unit and use an external temperature control/termocouple, and just have the analogue voltage turned all the
way up for my desired result?
[Edited on 19-5-2014 by JefferyH]
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Bert
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Since you don't specify actual equipment, that is a bit tough to answer-
But MY personal PID setup are all stupid simple. Temperature sensed is too low, relay turns on, full voltage supplied to heating equipment. After a
couple of cycles, controller learns response time of system it's running and anticipates both off and on points, preventing over and undershoot. NO
proportional control of voltage or current supplied, it's just a very clever on/off switch.
Look here:
I just want to buy a controller and hook it up!
http://castboolits.gunloads.com/showthread.php?115724-Projec...
From the ground up! For theory, engineering concepts & breadboard design-
http://www.mae.ncsu.edu/silverberg/piddocs.pdf
[Edited on 19-5-2014 by Bert]
Rapopart’s Rules for critical commentary:
1. Attempt to re-express your target’s position so clearly, vividly and fairly that your target says: “Thanks, I wish I’d thought of putting it
that way.”
2. List any points of agreement (especially if they are not matters of general or widespread agreement).
3. Mention anything you have learned from your target.
4. Only then are you permitted to say so much as a word of rebuttal or criticism.
Anatol Rapoport was a Russian-born American mathematical psychologist (1911-2007).
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Rogeryermaw
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controlling the temp of your reactions is one of the most basic lab skills. you can use infrared to coarsely monitor the plate temp, thermometer in
your oil bath and in your reaction flask and one at the still head for distillation/ fractional distillation. mastering these skills before shelling
out a kings' ransom for star wars equipment will serve you far better and for a lifetime.
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JefferyH
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| Quote: Originally posted by Bert | Since you don't specify actual equipment, that is a bit tough to answer-
But MY personal PID setup are all stupid simple. Temperature sensed is too low, relay turns on, full voltage supplied to heating equipment. After a
couple of cycles, controller learns response time of system it's running and anticipates both off and on points, preventing over and undershoot. NO
proportional control of voltage or current supplied, it's just a very clever on/off switch.
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Yes but the problem with this is such a thing would be very trouble some for controlling the speed of a vacuum distillation, especially if it needs to
be made to go slow and not on full blast.
For such a thing I found:
http://www.glascol.com/product/subproduct/id/219
Can control temperature and voltage, so you can control the speed at which it heats up to set temperature by varying the voltage. Such a thing would
be perfect for any sort of distillations where speed needs to be controlled, or in general for when temperature of the solution needs to be set.
I found another temperature controller for half the price but it doesn't have the option to control the voltage.
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Bert
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I'm going to say this one more time and then shut up.
Learn to run processes manually, using thermometer(s) and watching them like a hawk. It's as basic a skill to a chemist as driving a nail with a
hammer is to a carpenter.
That unit costs $1,100.00+. It is NOT your best use of (limited?) funds as an amateur chemist who's been registered here for about 2 weeks and is
asking the kind of questions you have so far.
Learning to make your own temperature and voltage controls WOULD be a valuable skill, I would endorse doing THAT. Buying fancy equipment only makes
you broke, not skilled. Only actual, hands on practice can make you skilled.
Oddly enough, a master in any craft tends to have skill with some very simple versatile tools, with which he can do a great deal on a "one off" basis
if given sufficient time and the needed materials... The fancy, expensive specialty equipment is more applicable over in production.
Rapopart’s Rules for critical commentary:
1. Attempt to re-express your target’s position so clearly, vividly and fairly that your target says: “Thanks, I wish I’d thought of putting it
that way.”
2. List any points of agreement (especially if they are not matters of general or widespread agreement).
3. Mention anything you have learned from your target.
4. Only then are you permitted to say so much as a word of rebuttal or criticism.
Anatol Rapoport was a Russian-born American mathematical psychologist (1911-2007).
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Varmint
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Sounds like a cook with unlimited funds in his near future, leveraging said funds against current investment.
And a hotplate with instantaneous temperature tracking from the heated surface?
I guess I have to move past this "junk" Corning 420D I've got because there is nothing "instant" about the way it works.
DAS
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JefferyH
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Bert,
You've given very sound advice and are correct, there is no reason to skimp on having the base knowledge. There's no argument there. Building various
circuits has always been a very prominent interest of mind. I have a very active engineering-mind set that I can't go a single day without coming up
with an idea for building something unique. The only problem is most of the time my ideas far exceed the knowledge required to create them. Which,
isn't really a bad thing, as it's one of the primary forces that drive me in life. The phenomena of chemistry and particle physics has always been a
fascinating endeavour for me, next to quantum physics, the study of space and time, most all fields of engineering, and alternative energy sources.
All of this is mostly self encompassing. My deepest yearning is to understand all there is to understand, as grandiose as that may sound. With my
continually growing interest in these fields, I would just hate to purchase something below parr only to be compelled by my desire to optimize
efficiency and end up buying something more expensive and feature-heavy anyways
While I do have more than enough [family] money to buy whatever I need, and plan to start up a research lab one day, taking a look at these PID units
you have linked has proved very interesting. I'd go for under $100 versus $1000 anyday, especially when the option for greater abilities of
customization is present.
Varmint,
I hope you don't throw around accusations like that to every aspiring chemist you encounter. Accusing someone of being a "cook" because of their
financial situation is beyond absurd. Feel free to apologize... It is shocking that having an interest in chemistry on this forum
of all places, can get somebody bullied.
Back on topic.... Varmint if you are interested in the hot plate I mentioned it is nothing more than a standard (100-250w) digital hotplate with
digital stirring, knobs controlling each. It's nothing impressive as far as heating ability is concerned. If you get one I would not waste your money
on a weak one or you will be disappointed. This one was around $250+ new and I believe for $100-$150 more I could have gotten one over double or
triple the power. While this one does have very responsive temperature feedback when stirring is employed, it's speed of heating has always left me
disappointed, even if cranked up maximally.
[Edited on 20-5-2014 by JefferyH]
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JefferyH
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Bert
I almost forgot to mention. When you mentioned the PID stuff I did some googling and found some very interesting things and many ideas have come to
mind. Now that I know how a thermocouple probe can be paired with a regulated and a heating element, I'm looking to a self contained condenser pump.
Instead of having ice cooling water pumping it into the condenser, it would be fairly easy to use a Thermoelectric Cooler to rapidly
cool the water in a closed container so that the condenser water never really warms up too much. This could be an awesome substitute to having a
bucket of ice water and a pump sploshing around in my lab.
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